Although polycrystalline silicon ingots are obtained by crushing polycrystalline silicon rods synthesized by a Siemens method or the like, since the polycrystalline silicon ingots as a raw material for production of CZ silicon single crystal are demanded to have high surface cleanliness, the polycrystalline silicon ingots are, after the crushing, subjected to chemical etching using nitro-fluoric acid or the like, for the purpose of removal of contaminants adhered on the surface, and after being cleaned with chemicals, subjected to a contamination inspection of the surface and the size classification, and packed as products. For example, Patent Literature 1 (Japanese Patent Laid-Open No. 2009-298672) proposes that as a technology of simply and securely preventing the unusual oxidation phenomenon called stains generated on the surface of crushed ingots of polycrystalline silicon during storage of the bagged crushed ingots, the crushed ingots of polycrystalline silicon are treated with a cleaning solution containing fluorine, washed with water, dried, and thereafter held at a temperature of 45° C. or higher for 20 min or longer in circulation of an inert gas whose dew point is −35° C. to −20° C. under reduced pressure.
Also until now, with respect to packing, in order not to contaminate the surface of polycrystalline silicon ingots, there have been made studies on materials for bags and the like to be used for packing; and due to recent year's improvements of analytical technologies, there has gradually been clarified the relationship between the level of the surface contamination of the polycrystalline silicon ingots and the production yield of CZ silicon single crystal.
Since bags for storing polycrystalline silicon ingots are required to be capable of maintaining cleanliness of the surface of the polycrystalline silicon ingots to be stored, materials for having a strength and an elongability in degrees not causing the bags to be broken, torn or otherwise by the polycrystalline silicon ingots having sharp irregularities on their surfaces are used, and linear low-density polyethylenes and the like are generally said to be preferable.
The linear low-density polyethylene (LLDPE) is produced, for example, by an ionic polymerization method, and the polymerization catalyst used at the time includes Ti-based, Cr-based, Zr-based catalysts and the like. From the viewpoint that the LLDPEs are those capable of maintaining cleanliness of the surface of the polycrystalline silicon ingots to be stored, the distribution of the composition and the molecular weight distribution of the LLDPE become important. Further in LLDPE, particularly, the intermolecular distribution of the comonomer concentration largely varies depending on the characteristics of the catalyst; the tensile strength, the heat sealability, hexane-soluble components and the like change; and the elution of low-molecular components are observed in some cases. Further in LLDPE, an antioxidant, an ultraviolet absorbent, a light stabilizer and an antistatic agent are used as additives, and their concentration ranges vary depending on the products.
Although these additives are expected to have not a little influence on the surface cleanliness of polycrystalline silicon ingots stored in a LLDPE bag, the degree of the influence has not been clarified so far.